a(n)=(s(n)+6)/9, where s(n)=n-th base 9 palindrome that starts with 3.

Terms

a(0) =1a(1) =4a(2) =28a(3) =29a(4) =30a(5) =31a(6) =32a(7) =33a(8) =34a(9) =35a(10) =36a(11) =244a(12) =254a(13) =264a(14) =274a(15) =284a(16) =294a(17) =304a(18) =314a(19) =324a(20) =2188a(21) =2197a(22) =2206a(23) =2215a(24) =2224a(25) =2233a(26) =2242a(27) =2251a(28) =2260a(29) =2270